Mast cells are the cellular nidus of allergic diseases and the cell responsible for disease in urticaria pigmentosa and systemic mastocytosis. This project is designed to ablate mast cells by attaching cytotoxic agents to IgE or antigen and selectively introducing the toxic product into mast cells. IgE linked to ricin kills RBL cells spontaneously and after crosslinking the IgE with anti-IgE. IgE linked to ricin's A chain kills only in the presence of monensin, a carboxylic ionophore. Therefore, IgE-linked immunotoxins are a new and possibly useful way to ablate mast cells.